The present disclosure relates generally to a refrigeration system and more particularly to a refrigeration system that uses carbon dioxide (i.e., CO2) as a refrigerant. The present disclosure relates more particularly still to a CO2 refrigeration system that controls an amount of subcooling of a CO2 refrigerant.
Refrigeration systems are often used to provide cooling to temperature controlled display devices (e.g. cases, merchandisers, etc.) in supermarkets and other similar facilities. Vapor compression refrigeration systems are a type of refrigeration system which provides such cooling by circulating a fluid refrigerant (e.g., a liquid and/or vapor) through a thermodynamic vapor compression cycle. In a vapor compression cycle, the refrigerant is typically compressed to a high temperature high pressure state (e.g., by a compressor of the refrigeration system), cooled/condensed to a lower temperature state (e.g., by rejecting heat to ambient air or another fluid in a gas cooler or condenser), expanded to a lower pressure (e.g., through an expansion valve), and evaporated to provide cooling by absorbing heat into the refrigerant. CO2 refrigeration systems are a type of vapor compression refrigeration system that use CO2 as a refrigerant.
Heat absorption and heat rejection are two of the four thermodynamic paths that make up the vapor compression cycle. Both heat absorption and heat rejection take advantage of latent heat transfer, causing a refrigerant to change state from a saturated liquid to saturated vapor (i.e., evaporation) or from a saturated vapor to a saturated liquid (i.e., condensation). As heat is absorbed or rejected during evaporation and condensation, the pressure and the temperature may remain constant (this may not be the case if the refrigerant is a blend of refrigerants that exhibit different saturation characteristics). Any heat transfer that occurs outside of this phase changing process is known as sensible heat transfer and results in a change in temperature of the refrigerant. Sensible heat transfer can be defined as either a subcooling of liquid or a superheating of gas. When pressure is constant and the temperature of a refrigerant decreases below its saturated temperature at that pressure, its subcooling value increases. Likewise, when pressure is constant and the temperature of the refrigerant increases above its saturation temperature at that pressure, its superheating value increases. Alternatively, if the temperature remains constant, subcooling and superheating can be achieved by either increasing the pressure of the refrigerant above its saturation pressure at that temperature or decreasing the pressure of the refrigerant below its saturation pressure at that temperature, respectively. Some refrigeration systems seek to achieve a subcooling setpoint by increasing the pressure of a refrigerant to be greater than its saturation pressure. However, a refrigerant not in a subcritical region (i.e., having a temperature above the critical temperature of the refrigerant) does not have the capability of latent heat transfer (condensing or evaporating) and thus cannot be condensed isothermally by increasing its pressure. Therefore a refrigerant having a temperature greater than its critical temperature has no corresponding saturation pressure. For this reason, it is common for non-subcooling control schemes (such as methods to maximize system COP) to be implemented to control the high side of supercritical vapor compression cycle systems.
This section is intended to provide a background or context to the invention recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art and is not admitted to be prior art by inclusion in this section.